The present invention relates to a vacuum pump which is extremely small in size and light in weight.
A conventional small-sized vacuum pump is constructed as shown in FIG. 10. A casing 120 of the vacuum pump is divided into a pumping chamber 121, an operational chamber 122 and piston rod bearing chamber 123. In one side wall of the pumping chamber 121, an intake valve 124 and a discharge valve 125 are provided so as to cooperate with reciprocal movements of a piston 126. The piston 126 is fixedly secured to one end of a piston rod 127.
An armature unit 128 is fixedly attached to the piston rod 127 located within the operational chamber 122. The armature unit 128 cooperates with an electromagnet M provided within the casing 120. The electromagnet M is comprised of a core C and a coil 129. When the electromagnet M is energized, the armature unit 128 is attracted by the core C. In accordance with the rightward movement of the piston 126, air confined in the pumping chamber 121 is discharged via the discharge valve 125 outside the chamber 121. When, on the other hand, the electromagnet M is deenergized, the rod 127 is moved leftwardly due to restoring force of a spring 130 which is provided over the rod 127 and is interposed between the armature unit 128 and a right hand side wall of the operational chamber 122. When the piston 126 moves leftwardly, air in a chamber (not shown) to be evacuated is sucked via the intake valve 124.
In order to energize the electromagnet M, a pulsating current or a half-wave rectified a.c. current is flowed in the coil 129. To this effect, an a.c. power source or d.c. power source has been used. For the a.c. power source, a commercial a.c. power source of either 50 Hz or 60 Hz is used. When using the d.c. power source, the pulsating current is produced while undergoing switching actions with respect to a d.c. current flowed from the battery.
FIG. 11, there is shown a block circuit diagram illustrating an electric system using the battery. An oscillation circuit 151 outputs pulse trains of a predetermined frequency. A switching circuit 152 receives such pulses and undergoes switching actions in accordance with the pulses fed from the oscillation circuit 151 with respect to a d.c. ccurrent supplied from a battery 153. Thus, the pulsating current is flowed into the coil 129, whereupon the electromagnet M is intermittently energized.
As described, the piston 126 reciprocally moves back and forth due to the attraction force exerted to the armature unit 128 by the energization of the electromagnet M and the restoring force of the spring 130. In the conventional vacuum pump, since the air in the vacuum chamber has been sucked into the pumping chamber 121 by the restoring force of the spring 130, biasing force of the spring 130 must be large for ensuring sucking. Therefore, such spring is generally large in size. On the other hand, air in the chamber 121 is discharged by th energization of the electromagnet M against the biasing force of the spring 130 and therefore, a large size electromagnet is needed to overcome the spring force. As a result, the overall arrangement of the vacuum pump becomes large in size despite a demand in reducing the size of the vacuum pump.
Further, notwithstanding the fact that a requirement exists such that the gap between the outer peripheral surface of the piston 126 and the inner peripheral surface of the chamber 121 be minimized in order to sealingly perform pumping actions, the gap needs to be formed therebetween to allow piston 126 to be smoothly moved. Since these two conflicting requirements are compromised, the sealing of the gap therebetween is not perfect and thus pumping actions cannot be efficiently implemented.
The tip and rear portions of the piston rod 127 are slidingly movably supported by a bearing portion 131 and by the piston rod bearing chamber 123. The piston rod 127 thus supported does not generally rotate about its axis, so that the contacting portions of the piston rod 127 and the bearing portions 131 and 123 are locally abraded. As such, the service life of the piston rod 127 is shortened due to the local frictional wearing.
The circuit shown in FIG. 11 also involves the following disadvantages. The disadvantages are caused by the fact that the waveform of the pulsating current flown in the coil 129, i.e. frequency and duty ratio, is determined depending upon the fixed output from the oscillation circuit 151. The battery voltage is initially high but is gradually lowered as it is used, and the amount of work executed by the armature unit 128 is proportional to an integration value of the current flowed into the coil 129. Accordingly, provided that the duration of the armature unit energization is constant, the amount of work is reduced if the current level is lowered. Reversely, the amount of work is increased if the current level is raised. In this manner, the level of the current changes depending upon the change n the battery voltage, and depending upon the change in the current level; and the amount of work executed by the armature unit 128 changes greatly. Therefore, the suction pressure and discharge pressure of the pump is lowered as time passes, efficiency of the pump cannot be maintained and the operation of the pump becomes unstable. If the vacuum pump is set to perform predetermined operations with a criteria of a relatively lower voltage of the battery, the stroke of the piston is caused to be excessively long in the range of a higher voltage of the battery.
In addition, while it is necessary that when the armature unit moves backward due to the restoring force of the spring 130, the restoring force and the suction or discharging pressure must be balanced. If the discharge pressure is lowered due to the change in load, the return stroke of the armature unit becomes excessively short, whereas when the discharge pressure is increased, the return stroke thereof is excessively long due to a backup pressure of the discharge valve. In this manner, if the stroke of the armature unit 128 is excessively long, the end face of the armature unit 128 impinges upon the side wall of the operational chamber 122, whereby noisy sound and heat are generated. Moreover, the service life of the armature unit is shortened and troubles are liable to occur. Power loss is also caused due to extra work. Where the pump is used under a condition where the pump installation is inclined, and/or used in a circumstance where temperature change and vibrations exist, the pumping action is thereby greatly influenced. For such reasons, an allowable using condition or the purpose for using the pump is restricted. Furthermore, the vacuum pump cannot be made small.